The invention relates to a method according to the preamble of the patent claim 1, and a device for carrying out the method.
The propeller drive dominates as drive for airplanes having cruising speeds of below about 300 km/h. These airplanes are, in the military sphere, especially transport airplanes and propeller-driven remote-controlled missiles, and in the civil sphere, sports planes and business cruise airplanes. In the civil sphere, the noise emission of these airplanes is seen as a nuisance by the population in the vicinity of airports, not least owing to the large number of flight movements, and to the often low flying altitude.
With smaller propeller airplanes of general aviation, the important noises are emitted by the engine exhaust and by the propeller. The noise emission from the engine exhaust is normally reduced by sound muffling.
In order to diminish the noise emission, which is based mainly on the propeller noise, it is known in a method according to the preamble of the patent claim 1, to generate, and thereby extinguish, the propeller sound field by superposition of the sound fields of antisound generators of nth order, which are arranged in a fixed position to the rotor shaft, the generation being done in antiphase in terms of wave geometry and spectral distribution of direction. In this connection, separate antisound generators are employed to simulate the sound fields, for example, a pneumatically driven siren, whose rotor runs synchronously with the propeller. It is further known to construct a part of the airplane wing skin as membrane of an electrodynamic loudspeaker in the capacity of antisound generator. Such additional antisound generators are expensive and difficult to realize (DE-Offenlegungsschrift No. 2,009,105).
Since the noise emission of the propeller is especially dependent on the blade-tip Mach number, it is known to decrease the generation of noise by reducing the blade tip Mach number. A decrease of the blade-tip Mach number is necessarily connected with a loss of thrust. Accordingly, additional precautions have to be taken in order to compensate this loss of thrust. Generally, these measures involve changes to the propeller and/or the arrangement of a gear between engine and propeller. Accordingly, this approach, too, is generally expensive.